Frequency modulation receiver



May 5, 1942. w. VAN-B. ROBERTS 2,282,092

FREQUENCY MonuLATIoN RECEIVER Filed March 26, 194.1

\ y-IHIIIP k Q 1Km l INVENToR T'roRNEY Patented May 5, 1942 FREQUENCY Moncm'rroN RECEIVER Walter van B. Roberts, Princeton, N. Jv., assignor of America, a corporation to Radio Corporation of Delaware Application Maren 2s, 1941, serial No. 385,246

' s claims. (Cl. 25o-2o) My present invention relates generally to a method of receiving frequency modulated carrier waves, and more particularly to a novel and improved receiver of the superheterodyne typeadapted to receive frequency modulated carrier y waves. t

The usual receiver of frequency modulated waves employs a pair of rectiiiers whose outputs are connected in opposition. Thus, when the frequency is at its mean, or carrier, value the outputs of the two rectiiiers cancel so that amplitude variations of the carrier do not produce any direct current output. However, when the incoming waves are of different frequency from their mean -value the cancellation is no longer complete, and amplitude variations reaching the rectiers produce unwanted variations in the combined output circuit of the rectiiiers. 'I'his susceptibility to disturbance from amplitude variations occurs whenever the Wave frequency is varied; that is, most of the time. Hence, in present receivers an attempt is made to eliminate amplitude variations from the received waves before these are impressed on the rectiers. Devices used for this purpose are generally known as limiters, and the function of a limiter is to limit the amplitude of a wave impressed there-v on to a xed value. Unfortunately, however, the mere limiting of amplitude is not sufficient to maintain the amplitude of the voltage impressed on the rectiflers independent of amplitude variations of the incoming signals. This is because tuned circuits are interposed between the limiter and the rectlers, and the amplitude of the volt- .age developed in these tuned circuits depends not only upon the amplitude of the current pulse amplitude of the wave.

frequency deviation is usually less than kilocycles, the attainment of this condition re-iy Waves by producing from each wave a fixed quantum of electric charge regardless of the 'I'he average flow of current produced by these quanta is, therefore, directly proportional to the frequency of the waves' and independentk of their amplitude. It will be evident that to produce a large percentage variation in the aforesaid average current, the mean frequency of the waves must be so low that thefrequency variation due to modulation will be comparable to the mean frequency.

This condition is easily enough obtained in a superheterodyne type of receiver by making the mean, or carrier, value of the intermediate frequency only a little greater than the frequency deviation produced by modulation. Since the a hundred quires using an intermediate frequency of the order oione or two hundred kilocycles (kc.) with the result that image interference is difficult to avoid. Thus, the counter type of FM (frequency modulation) detectoris, also, subject to drawbacks when employed in connection with the usual type of superheterodyne receiver.

Another important object of my invention,

therefore, is to utilize the aforesaid counter type of FM detector in a superheterodyne receiving system in which two frequency conversions take place, the same local oscillator being employed for each conversion; the cascade conversion resulting in the production of only one primary image frequency of the signal with respect to the rst heterodyne, this single image frequency being the same as the second intermediate freemanating from the limiter, but, also, upon the waveshape of this pulse and, hence, upon the amplitude of the incoming signal Wave.

vIt can, therefore, be stated that it is one of the main objects of my present invention to provide an improved method of, and means for, converting frequency modulated carrier waves having undesired amplitude variations into a rectified current whose direct current component varies directly with the frequency of the collected modulated carrier waves but does not vary with amplitude variations of said waves.

In my application Serial No. 354,982, led August 31, 1940, there are disclosed and claimed various arrangements for receiving frequency modulated carrier waves which seek to overcome quency.

Still other objects of the invention are to improve the reception of FM waves, and more especially to provide FM receivers which are reliable, eiiicient and economically manufactured and assembled.

'I'he novel features which I believe to be characteristic of my invention are set forth in particularity in the nappended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have inthe aforementioned drawbacks. Each of the arrangements operates on the principle of a countdicated diagrammatically a circuit organization vhtereby my invention may be carried into ef- Referring, now, to the accompanying drawing, there is shown a superheterodyne receiving system adapted to receive FM waves, or in general "timing modulated carrier waves. The presput electrodeent FM reception band covers 42 to 50 :megacycles (mc). Let it be assumed that dipole signal collector I collects-FM waves. and impresses, thefwave energy upon the tunable input circuit 2. 'I'he latter includes a variable tuning condenserf 3 for adjusting lthe frequency of the circuit over the 42-50 mc. range. The 'FM waves,

whose center frequency coincides with the frequency of circuit 2, are applied to the signal in- 4 of the first converter tube 5. The cathode of the latter may be connected to ground through a self-bias resistor 8 shunted by a carrier by-pass condenser. The plateelectrode Ijisconnected to the r.primary winding of transformerl. An oscillation input electrode III is located between the positive screen of tube type of FM detector disclosed as Fig. 6 of my 5 and cathode. The low potential side of inputv r circuit 2 is returned to ground for signal currents by the condenser I I.

`Local oscillations are applied to electrode III by a local oscillator comprising a tube I2 having its control grid regeneratively coupled, as at yM, to the tunable tank circuit provided with the adjustable tuning condenser I3.

cuit is in the plate to cathode circuit of tube I2, and the oscillator will be recognized as of a conventional construction. Adjustment of condenser I3 varies the frequency of the tank circuit over a range such as to produce afpredetermined range of beat frequencies atthe input circuit 2' of the second converter tube 5'. .For-example, the oscillator tank circuit may be adjusted over a frequency range of 20.9 to 24.9 mc.v In

that case', the variable tuning condenser-3' would be adjusted to tune the input circuit 2 over a range of 21.1 to 25.1 rnc.

The second converter tube 5' may be con- I structed exactly like tube 5. Hence, corresponding elements bear the same reference numeral,

except that in the circuit of tube 5' the numerals" are given a prime designation.' The local oscillations produced at the oscillator tank circuit are applied over lead to the oscillation input electrodes I0 and I0. The'lower end of lead 20 terminatesin an adjustable tapgslidable along the coil of the oscillator tank circuit. The plate circuit of the second converter. tube includes a resonant .output circuit 2| tuned'to a fixed frequency of 0.2 mc. 'I'he variable condensers 3, 3'

and I3 will, of course, preferably be uni-conf trolled in any of the well known ways for producing satisfactory'tracking of the circuits. The dotted line 22 denotes the usual mechanical adjustment device for varying the positions of the rotors of condensers 3, 3 and I3 in unison. The second I. F. value is a constant, and is much lower than the first variable I. F. value.

The second I. F. voltage is applied to the input circuit voi the I. F. amplier 30. The latter will provide predetermined amplification of the FM waves at the secondfI. F. of 0.2 mc. 'I'he amplified FM waves, at the second I. F. value, are applied subsequently to a limiter stage 3i. The latter is schematically represented,I since those skilled in the art well known how to construct such a network. Briefly, the limiter operates in the mannerof a saturated ampliiler, and has an input-output characteristic as shown in the rectangle 3I. The limiter functions to minimize amplitude variations in the FM. carrier. Usually it grid circuit includes a resistor, shunted by an I. F. by-pass condenser, across which is developed a uni-directional voltage vproportional to carrier amplitude. The voltage may be applied over lead 40, including appropriate' aforesaid application. While the invention is by no means restricted to such detector, it is shown to illustrate the present system. The limiter 3| feeds the limited FM signals to the input electrodes of screen grid amplifier tube II. The plate circuit of tube 6I includes a resistor 82. The plate end of resistor 62 is coupled by condenser 63 to the upper end of resistor M.

' Thediode 65 is connected between the upper The tank cir- 40, voltage.

end ofresistor 84 and its grounded end. A second diode .66 has its anode connected to the anode of diode 65 through resistor 61, while the anode of diode 66 is further connected to the input grid of the following audio amplifier tube .(not shown) through resistor 1I. The cathode of diode 66' is at ground potential. Theplate circuit of the following audio amplifier tube, properly by-passed, may feed the detected audio. voltage to any desired type of reproducer.

The plate resistor 62 develops a square wave of voltage, while condenser 63 has passed through it a definite quantum of charge at each v half cycle of the square wave voltage. The resistor 64 develops a voltage from the passage of the aforesaid charge in one direction, and the following audio amplifier tube is energized by this The pair of diodes 6.5--66 are arranged in combination with resistors 61-1I to form a polarity lter to permit the transfer of negative potential pulses from resistor 64 to the audio amplifier, but to suppress positive potential pulses. The constants are so chosen as to permit the system to assume steady state conditions within the time during which the flattopped plate potential wave stays constant at a limiting value.

If E is the maximum potential change across resistor 62, and F the frequency of the limited resistor feeding the audio circuit through an audio filter network. In such case, as before, an increase in the repetition rate of the FM waves applied at the input of tube 6I will increase the number of current pulses per unit of time, and, therefore, increase the average value' of current flowing through the ,output resistor. Conversely, a decrease in repetition rate decreases the average current through the output resistor, and the average potential across the latter is a perfectly linear function of the frequency of the input signal.

Returning, now, to the question of image freu', vsriabxy to has the ami casacca quency, it will be seen that when the frequency of the local oscillator is 20.9 mc., the only fre quency, other than the desired 42. mc. carrier, which is capable of producing, by beating with the oscillator, the first intermediate frequency value of 21.1 mc., is the frequency 0.2 mc. which by addition to the oscillation frequency will produce the first intermediate frequency value. Similarly, when the oscillator is adjusted to 24.9 mc. and the signal is 50 mc., the nrst intermediate frequency value being 25.1 mc., the only im age frequency is again 0.2 mc. It can, in fact, be demonstrated that for any signal in the tuning range, the only image interference that can be caused is that which occurs at the single fre-y quency of 0.2 mc. However, this latter frequency is so lowl when the second intermediate frequency value is chosen as in the present illustration, that the short wave' dipole antenna I, is very little responsive to the image frequency, and the antenna tuned input circuit 2 is far off-tune from the image frequency in addition. 'Furthermore, and as shown in the drawing, it is a simple matter to insert one or more wave traps i' at .the antenna circuit which will further greatly at tenuate interference at the particular image frequency without appreciably affecting the signal frequencyv response, since the difference is so great between the frequencies involved;

While the value of 0.2 mc. has been given as' illustrative, of course other values may be used in accordance with the invention. What is essential in the present invention is that the second intermediate frequency value be so low that the frequency variations due to modulation be comparable to the intermediate"y frequency value, but not so low that the spectrum of the modulated wave contains any considerable frequency components as low as the upper limit of audibility. Of course, the lower the second inter-- mediate frequency value can be made, without introducing undesirable effects of. any sort or without increasing circuit design diiculties or expense, the better, as the result is Aincreasedaudio output for a given frequency modulation.

While I have indicated and described a system for carrying my invention into effect, it will be apparent to one skilled in the art that my inven-l tion is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims. 4

What I claim is A 1. In a receiver. of timing modulated carrier waves, a first converter networkv having a signal input circuit and a signal output circuit, means for tuning said input circuit over a range of desired carrier frequencies, a local oscillator including means for varying the frequency thereof over a range of frequenciessubstantially different from .the frequency range of said input circuit, the said output circuit including means for tuning it over a range of frequencies which represents the difference between said input frequency range and said oscillator frequency range, means for pplying oscillations from said oscillator to said converter, `a second converter provided with input electrodes coupled to said first converter output circuit, said second converter having an output circuit, means for applying oscillations from said oscillator to said second converter, said second' converter output circuit being iixedly tuned to a constant frequency which is equal to the difference between the frequencies of said oscillator and the output'ofl said first converter, means for producing from said modulated waves a fixed quantum of charge, means for integrating said charges to produce a flow of current proportional to the frequency of said waves and independent of theiramplitude, the frequency of said oscillator, at any point in its range, being such that twice its frequency is less than the frequency of a' desired carrier wave by an amount` which-is greater than, but Vgreater only by an order of magnitude or less, than the maximum frequency deviation of the desired waves from their center frequency.

2. In a superheterodyne receiver of the frequency modulation reception type, at least two converter stages arranged in cascade, each of said stages being provided with' an input circuit I and an output circuit, a local oscillator provided with a resonant tank circuit, means for tuning .the first converter input circuit over a range of desired modulated carrier waves, means for adjusting the frequency of said tank circuit over a range of oscillation frequencies which are 'substantially different from the desired carrier frequency ange, means for applying oscillations from said oscillator to both said converters; means for tuning the input circuit of the second converter through a range of frequencies equal to the difference between the carrier frequency range and the oscillator frequency range, said oscillator frequency range being chosen to have said second converter input circuit frequency range different from the oscillator frequency range by a relatively small constant frequency difference, said second converter output circuit being tuned to said small frequency difference, means for producing from said modulated waves a xed quantum of charge, means for integrating said charges to produce a flow of current proportional to the frequency of said waves and independent of their amplitude, and means for simultaneously adjusting the tuning of said first converter input circuit, said second converter input circuit and said oscillator.

3. A receiver of frequency modulated carrier waves comprising a source of signal waves, a local oscillator, a first converter stage having connections to said signal source and oscillatorysaid converter having an output'circuit tuned to the difference of frequency between signals from said source and oscillations from said oscillator, a

second converter stage having a signal input cou- Arpling circuit to said first converter output cir.

cuit and also provided with an output circuit,

y means for applying oscillations from said oscillator to said second converter, said second converter output circuit being tuned to the difference frequency of voltages produced in said first converter output circuit and the frequency of oscillationsffrom said oscillator, means for generating from carrier waves produced in said second converter output circuit a fixed quantum of charge, means for integrating said charges to provide a flow of current proportional to the frequency of saidwaves and independent of their amplitude, and the frequency of said oscillator being so chosen that twice its frequency is less than the frequency of the desired signal waves by an amount which is greater than, but greater only by an order of magnitude or less' than, the

maximum frequency deviation of the signalV constant frequency is greater than, but only a few times greater than, the maximum frequency stantiallv different from the desired carrierfredeviation of said waves. combining said oscillations with said waves to produce waves of a ilrst intermediate frequency, combining said same oscillations with said first intermediate frequency waves to produce waves of a second intermediate quency range. means for applying oscillations from said oscillator to both said converters. means for tuning the input circuit of the second converter through a range of( frequencies equal to the difference between the carrier frequency range and the oscillator frequency range,

frequency whose mean frequency is said constant frequency, utilizing said second intermediate frequency waves to produce xed and equal quanta of charge displacement from each of said waves, accumulating the charge displacements in one direction to provide a current having a direct component proportional to the frequency of the waves of said second intermediate frequency but independent of the amplitude thereof, and utilizing the variations of said direct'component to reproduce signals represented by the frequency modulation of said first named waves.. Y

5. In a receiver of frequency modulated carrier waves, a rst converter network having a signal input circuit and a signal output circuit, means for tuning said input circuit over a range of desired carrier frequencies, a local oscillator including means for varying the frequency thereof over a range of frequencies substantially different from the frequency range of said input circuit, the said output circuit including means lator to said converter, a second converter provided with input electrodes coupled to said first converter output circuit, said second converter having an output circuit, means for applyingoscillations from said oscillator to said second converter, said second converter output circuit being 0 xedly tuned to a constant frequency which is equal*to the ,difference betweenthe frequencies yof said oscillator and the output'ofsaid first converter, means for demodulating said moduany point in its range, being such that twice its frequency is less than the frequency of a desired i carrier wave by an amount which is greater than,

.but great/er only by an order of magnitude or vlated waves, the frequency of said oscillator, at 45 less, than the maximum frequency deviation of 540 the desired waves from their center frequency.

6. In a superheterodyne receiver of the` frequency modulation reception type, at least two converter stages arranged in cascade, each of said oscillator frequency range being chosen to have said second converter input circuit irequency range different from the oscillator frequency range by a relatively small constant frequency difference. said second converter output circuit being tuned to said small frequency difference, means for demodulating said modulated waves, and means for simultaneously adjusting the tuning 'of said first converter input circuit. said second converter input circuit and said'oscillator. l- Y 7. A receiver of frequency modulated carrier waves comprising a source of signal waves, a local oscillator, a .first converter stage having connections to said signal source and oscillator. said converter having an output circuit tuned to the difference of frequency between signals from said source and oscillations fromsaid oscillator. a second converter stage having a sig nal input coupling circuit to said first converter output circuit and also provided with an output circuit, means for applying oscillations from said oscillator to said second converter, said second first converter output circuit and the frequency -I of oscillations from said oscillator. means for deriving from carrier waves produced in said second converter output circuit modulation signals, and the frequency of said oscillator being so chosen that twice itsfrequency is less than the frequency of the desired signal waves by an amount which is greater than, but greater only by an orderV of magnitude or less than, the maximum frequency deviation of the signal waves from' their mean frequency.

8.l The method of receiving and demodulating timing modulated waves which comprises generating local oscillations whose frequency is half the difference between the mean frequency of said waves and a constant frequency which constant frequency is greater than, but only a few times greater than, the maximum frequency deviation of said waves, combining said oscillasaid stages being provided with an input circuit and an output circuit, a local oscillator provided justing the frequency of said tsank circuit over 00 a range of oscillation frequencies which are sub'- tions with said waves to produce waves of a first intermediate frequency, combining said same oscillations with said first intermediate frequency waves to produce waves of a second intermediate frequency whose mean frequency is said constant frequency, and utilizing said second intermediate frequency waves to produce signals represented by the frequency modulation of said first named waves.

i WALTERl vsn-B. ROBERTS. 

